Stainless iron and method of producing the same



April 25, 1933. w KEEN 1,905,721

STAINLESS IRON AND METHOD OF PRODUCING THE SAME Filed Aug. 14, 1930 Snnentor William H. Keen (lttornegs Patented Apr. 25, 1933 UNITED STATES PATENT OFFICE WILLIAM H. KEEN, OF ALBANY, NEW YORK, ASSIGNOR TO CHAS. W. GUTTZEIT, OF NEW YORK, N. Y.

STAINLESS IRON AND METHOD OF PRODUCING THE SAME Application filed August14, 1930. Serial No. 476,456.

The present invention relates to iron or steel alloys of the type commonly referred to as stainless iron and has for an object to provide an improved alloy of this char acter which has the property of free machining combined with corrosion resisting properties and to provide an improved method of treating such alloys to produce the desired properties.

The requirements of high speed production make it important that iron. or steel should be readily machinable. Many attempts have been madeto produce a stainless iron of good machinability by adding sulphur to the composition but the results obtained have been at the expense of the corrosion resisting properties.

The present invention provides a stainless iron which while retaining the usual corrosion resisting properties is also capable of being machined readily and effectively.

This result is obtained by a particular treatment of a stainless iron alloy of a usual composition such as one containing, 10 to 15% of chromium with carbon within a range approximating 0.06 to 0.15%. The treatment involves working (rollingv or forging) at a relatively low temperature and then preferably annealing to provide the so desired degree of hardness. The resulting iron or steel has a certain characteristic structure as shown by microscopic examination and will machine freely while retaining the usual corrosion resisting qualities.

The accompanying drawing forming a part hereof is a copy of a photo-micrograph showing the structure of an illustrative stainless iron of the character described treated in accordance with the invention.

' 40 The treated iron has the appearance of and is believed to be a mixture of sorbite and troostite, principally sorbite. When etched in concentrated hydrochloric acid and photographed at a magnification of 250 diameters the stainless iron has the appearance of a conglomerate mass of black and white particles regularly distributed and having the appearance of a mixture of small irregular chips of steel imbedded in a black matrix. It is this type of structure which introduces the proper amount of brittleness to machine freely into fine cuttings.

Ordinarily stainless iron or steel of the type to which this invention relates is hot rolled or forged at a temperature approximating 2100 to 2200 F. and sometimes higher. In accordance with the present invention the iron or steel is rolled 'or forged at a substantially lower temperature preferably between 1800 and 1900 F. and es- '0 pecially it is desirable that the temperature should not exceed a maximum of 1950 F. to prevent the formation of austenite.

After rolling at the indicated temperature the iron or steel is annealed and hardened to provide the desired hardness. To accomplish this result, the rolled or forged iron or steel may be simply heated to a temperature approximating 1500 to 1600 F. and allowed to cool in still air, or preferably to get better uniformity and no hard spots, the steel may be pipe annealed for approximately two to three hours at approximately 1560 F. allowing it to cool off to about 900 to 1100 F. and it may then be reheated to a temperature approximating 1525 to 1650 F. according to the hardness desired and allowed to cool'in still air from this temperature.

The best results as regards machinability combined with corrosion resistance have been obtained with stainless iron containing from 10.75 to 12.25% chromium and 0.1 to 0.15% carbon. 7

The structure obtained is remarkably free 5 machining even when machined at a surface speed of as much as 200 surface feet per minute. When threading operations are required the material should be somewhat ha der than for ordinary machining. In

to the hardness to be expected from a variation of the reheating temperature and will illustrate the close control required to obtain the hardness specified.

Temperature Brinell hardness 1525 F 179 1550 F 187 1575 F- 217 1600 F 235 1625 F 255 1650 F 293 When treated in the manner substantially as described the structure is of such a conglomerate nature that even at a Brinell hardness of 321 it will machine freely enough for use in automatic machines. A hardness of 248 to 302 is usually preferable for free'machining.

The importance of a low rolling temperature, presumably to avoid any possibility of producing a structure of an austenitic nature, cannot be overemphasized. Also for best results the desirability of close control of the temperature of the pipe anneal at 1560 F. should be emphasized. This heat should if possible be held to plus or minus 20 F. in order to obtain the structure described, in' fact the closer it is kept to 1560 F. the better the results obtainable.

The free machining qualities depend on physical characteristics and close observance of the temperatures is of the utmost importance. The structure of the iron obtained by rolling iron of the composition above indicated at the lowertemperatures as above indicated is quite different from the structure -of regularly annealed stainless iron. Such structure cannot be obtained by heat treatment by quenching and drawing in the usual manner.

The foregoing particular description is illustrative merely and is not intended as defining the limits of the invention. The treatment may advantageously be applied to stainless iron or steel of a wider range of composition than that here indicated including for example compositions having sulphur added to improve the machinability.

I claim:

1. A stainless iron or steel alloy combining approximately 10 to 15% chromium and approximately 0.1 to 0.15% carbon said alloy having a structure consisting essentially of a mixture of sorbite and troostite by virtue of previous working at a temperature approximating 1800 to 19009 F.

2. A stainless iron or steel alloy combining approximately 10 'to 15% chromium and approximately 0.06 to 0.15% carbon said alloy having a structure consisting essentially of a mixture of sorbite and troostite and martensite by virtue of previous Working at a temperature approximating 1800 to 1900 F. and annealing at a temperature approximating 1560 F.

3. The method of treating stainless iron or steel containing chromium in a proportion approximating 10 to 15% and carbon in a proportion approximating 0.06 to 0.15% to produce a structure consisting essentially of a mixture of sorbite and troostite which comprises working the same at a temperature approximating 1800 to 1900 F. and below 1950 F. as a limit and thereafter annealing by heating to approximately 1500 to 1650 F.

4. The process of treating stainless iron containing from 10.75 to 12.25% chromium and from 0.06 to 0.15% carbon to produce a readily machinable product which comprises heating the iron to a temperature of approximately 1800 to 1900 F., working at this temperature, thereafter annealing and hardening by heating to and maintaining at a temperature of approximately 1560 F. for two to three hours, allowing the iron to cool to 900 to 1100 F. and reheating to a temperature approximating 1525 to 1650 F. and allowing it to cool in still air.

5. The process of treating stainless iron containing 10 to 15% of chromium and 0.06 to 0.15% of carbon to produce a readily machinable product which comprises heating the iron to a temperature approximating 1800 to 1900-F. but below 1950 F. as a limit, working at this temperature, thereafter annealing and hardening by heating to and maintaining at a temperature of approximately 1560 F. for a considerable time, allowing the iron to cool to approximately 900 to 1100 F. and reheating to a temperature approximately 1525 to 1650 F. and allowing it to cool in still air.

6. The process of treating stainless iron containing 10 to 15% of chromium and 0.06 to 0.15% of carbon to produce a readily machinable product which comprises heating the iron to a temperature approximating 1800 to 1900 but below 1950 F. as a limit to prevent the formation of austenite, working by rolling or forging at this temperature, thereafter annealing and hardening by heating the rolled or forged iron to a temperature approximating 1500 to 1600 F. and allowing the iron to cool in still air.

' 7. The method of treating stainless iron,

or steel containing chromium in a proporture which comprises working the same at a 1 temperature approximating 1800 to 1900 F. and below 1950 F. as a limit, thereafter heating the same to a temperature approximating 1560 F. and cooling it slowly at least to a temperature approximating 1100 F. and thereafter reheating to a temperature approximating 1525 to 1650 F.

In testimony whereof I have aifixed my signature to this specification.

WILLIAM H. KEEN. 

